Current trends in gas turbine engines are moving towards so-called geared turbofan engines in which the fan is driven through a reduction gear train. The gear train allows the low pressure spool to be driven at higher rotational speeds which provides for a more efficient lighter engine core, whilst reducing the speed of the fan allows it to be a larger diameter thereby providing a higher bypass ratio. The reduction gear trains may be epicyclically configured where the fan is driven via the carrier of a planetary configuration, or a star configuration where the planet gears are fixed and the fan shaft is driven by the ring or star gear.
FIG. 1 shows a geared gas turbine engine 10 having a fan 12, low and high pressure spools, each having respective compressors and turbines driveably interconnected by respective shafts which are rotatable about a principal axis 11. Thus, there is a low pressure compressor 15 connected to the low pressure turbine 19 via a low pressure shaft, and a high pressure compressor 16 connected to a high pressure turbine 18 via a high pressure shaft. The low 15 and high 16 pressure compressors progressively compress air from an inlet downstream of a fan 12 to an outlet in flow proximity to the combustor 17. Compressed air flows from the high pressure compressor 16 to the combustor 17 in which fuel is added and the mixture burnt. The combusted gas then expands through and drives the high 18 and low 19 pressure turbines in flow series. The low and high pressure shafts interconnect the respective turbines and compressors provide the drive for the compressors.
The fan 12 is located at the front of the engine 10 to provide air for the inlet of the compressors and the main propulsive flow which is channeled down the bypass duct 22. The fan 12 is driveably connected to the low pressure shaft via a gear train 14 in the form of an epicyclic reduction gear box. The gear train 14 is located between the low pressure shaft and the fan 12 and is arranged to reduce the speed of the fan 12 relative to the speed of the low pressure turbine 19. Such an arrangement allows for a higher speed and more efficient low pressure turbine 19 together with and slow spinning larger fan which can provide a higher bypass ratio. This combination allows the speed of the fan and low pressure turbine to be independently optimised.
The fan 12 has a plurality fan blades 13 extending radially from a hub which is mounted so as to rotate about the principal axis of the engine 10. The fan 12 resides within a fan casing 21 which partially defines the bypass duct 22. An engine casing surrounds the engine core which comprises the low and high pressure spools and combustor 17. The engine casing generally provides containment and structural support for the engine core. The engine casing is ultimately attached to and supported by the wing of the aircraft via an appropriate arrangement of struts which extend across the bypass duct and the nacelle which attaches to a pylon as is well known in the art.
The gear train 14 is in the form of an epicyclic reduction gearbox which is driven in a planetary configuration. The gear train 14 includes a ring or annular gear which is held substantially stationary in relation to the engine casing, a planet gear set with individual planets gears interconnected via a carrier, and a sun gear. The sun gear is rotatably connected to the low pressure shaft. The fan 12 is connected to the output shaft of the gearbox which is in the form of the carrier of the planet gear via a fan shafting arrangement.
Generally, planetary gearboxes are used in power transmission systems across many industries including, for example: automotive, wind turbines, aerospace and marine. In its simplest form, it comprises a central gear or sun gear surrounded by multiple planet gears mounted on a single carrier, which in turn sits within a single ring gear which has internal gear teeth for engagement with the planet. The sun gear, carrier and ring gear are concentrically with the engine principal axis 11.
In operation, one of the sun gear, planet carrier and ring gear are held stationary with the other two providing an input and an output to the gearbox. This flexibility in selection of the various components as a stator or as an input or output rotors determines the gear ratio of the gearbox and allows for several drive variations, as are known in the art.
In epicyclic gearboxes which experience high loading, it is typical that the carrier forms a stiff structure around the gears to ensure efficient transfer of torque and reduce unnecessary gear wear. The axially opposed end walls of the carrier include apertures into which a so-called planet pin is inserted. The planet pin defines the central rotational axis of the respective planet gear and carries or includes the planet gear wheel bearing. During assembly the pins are fed into the apertures to locate the planet gears in the correct position in relation to the carrier. The pins are retained in the apertures, typically by a combination of interference fit and mechanical fasteners, such as bolts.
Under centrifugal loading the planet gear is forced radially outwards as the carrier spins. It is restrained by the planet pin via the planet bearing which becomes loaded on the radially inner surface, the radial inner surface being closest to the central axis of the carrier. The planet pin is restrained from moving by its fit within the carrier pin aperture in the axial wall which results in the radially outer portion of the carrier aperture carrying the load. However, it is desirable to provide the outer rim of the carrier with a reasonable shallow and consequently thin radial profile to reduce the centrifugal loading on the carrier and ring gear. The thin radial profile can result in a deflection under the centrifugally loaded planet pin which can result in significant deflection of the carrier plate and its rim. Further, the planet pin can be deformed by the centrifugal loading induced by the planet gear. The combined effect of these load induced deformations can be to move the planet gear radially outwards by an appreciable amount. Such a movement can affect gear meshing and deleterious levels of wear.
The present invention seeks to provide an improved planet pin and carrier for a planet gear.